e39 530 sport DISA valve problem?

[Jez49]

My engine just developed a nasty rattle, its coming from the little black plastic box attached to the inlet manifold and stops when the revs pass 4k rpm.

Does this sound like the DISA valve has broken / is giving up? Car still idles fine, noise literally started as I pulled in to the drive. Definately not tappet or injector noise.

Car is a 2000 e39 530 sport petrol on 130k miles

Thanks

Jez